ETA AQR (Eta Aquarii). Straddling the celestial equator is Aquarius's beloved four-star, Y-shaped "Water Jar" or "Urn," made of second-brightest Sadachbia (Gamma Aquarii) to the southwest, brightest fourth magnitude Zeta Aqr in the center, fifth magnitude northern Pi (the faintest), and at the eastern branch, number three in brightness, fourth magnitude (4.02) Eta. Physically, Eta is a white class B (B9) subgiant-dwarf that lies 185 light years away. The star's well- determined temperature of 11,400 Kelvin allows estimation of its modest ultraviolet light, which together with its visual radiation yields a luminosity 104 times that of the Sun and a radius 2.6 solar. All alone, no trace of a companion, the star carries 3.0 solar masses. Though the classification is in part that of a slightly evolved subgiant (which implies that hydrogen fusion has ceased, or is about ready to), Eta is right in the middle of its hydrogen-fusing phase, and is clearly a standard dwarf (which classification will be here adopted). Indeed, with an age of 175 million years, it is almost exactly half way through its 350 million year hydrogen-fusing lifetime. Eta's most outstanding characteristic is its high equatorial rotation speed of at least 245 kilometers per second (the actual value dependent on the unknown the tilt of its rotation axis), which gives it a rotation period of less than half a day (as opposed to 25 days for our Sun), a feature that is quite common for class A and B stars. The star's resulting simple spectrum (which is washed out by the Doppler effect) makes it ideal for the study of intervening interstellar gases, for which it is widely used. Eta's most interesting feature, however, involves Earth. The 26,000-year precession (wobble) of the Earth's axis slowly changes the pole star (now Polaris, in ancient days Thuban, and so on), the positioning of the plane of the Earth's equator, and the locations of the equinoxes and solstices. Precession (which is caused by the gravity of the Sun and Moon acting on the Earth's rotationally induced equatorial bulge) therefore continuously changes stellar positions (right ascension and declination, analogous to terrestrial longitude and latitude). In our astronomical era, the Water Jar is now moving northward across the celestial equator, from the sky's southern hemisphere to the northern. Positioned for "current transit" across the equator are Zeta and Eta. Zeta in fact made the transition in November of 2003. Next to go in the Water Jar will be Eta, which is slightly south of Zeta. So mark your calendars for October 1, 2022, when (within a day or so) Eta will become a new northern hemisphere star, giving us a fine excuse to have a precession party.
Written by Jim Kaler. Return to STARS.